To go along with the general pedantry: Velocity is being used incorrectly at times, I believe. It should be speed really when it's in relation to an object going around in circles, because velocity is a vector quantity which includes speed and direction of motion, e.g. 3 m/s East. Since the train is going round a circular track, even if the speed is constant the velocity is constantly changing because the direction is constantly changing. Speed by contrast is a scalar quantity, e.g. 3 m/s. </pedantic post>